Rotor winding shield for a superconducting electric generator

ABSTRACT

A generator rotor core ( 54 ) carrying superconducting windings ( 60 ) and having a shield ( 426 ) over the superconducting windings ( 60 ) to prevent external magnetic fields from impinging the windings. Axial shield edges ( 430/434 ) mate with corresponding features of the rotor core ( 54 ) or with structures affixed to or supported by the core ( 54 ) to support the shield ( 426 ).

FIELD OF THE INVENTION

The invention relates generally to superconducting generators andspecifically to a non-magnetic rotor shield for protectingsuperconducting rotor windings and optimizing the production of magneticflux generated by the rotor windings.

BACKGROUND OF THE INVENTION

An electric generator transforms rotational energy into electricalenergy according to generator action principles of a dynamoelectricmachine. The turning torque supplied to a rotating rotor by a combustionor steam-driven turbine is converted to alternating current (AC)electricity, typically three-phase AC, in a stationary stator thatsurrounds the rotor. The generator is a mechanically massive andelectrically complex structure, supplying output power up to 2,222 MVAat voltages up to 27 kilovolts. Electrical generators are the primarypower producers in an electrical power system.

As shown in the cross-sectional view of FIG. 1, a conventional electricgenerator 10 comprises a substantially cylindrical rotor 12 supportingaxial field windings or rotor windings 13. A direct current (DC)supplied to the rotor windings 13 produces a constant magnetic fluxfield that rotates with the rotating rotor within a stationary armatureor stator 14. One end 15 of the rotor 12 is drivingly coupled to a steamor gas driven turbine (not shown in FIG. 1) for providing rotationalenergy to turn the rotor 12. The opposing end 16 is coupled to anexciter (not shown) for supplying the direct current to the rotorwindings 13. An alternating current is generated in the stationarystator windings as the rotor's magnetic flux field crosses the statorwindings. Rotor rotation subjects the rotor 12 and the rotor windings 13to radial centrifugal forces that may result in radial distortion ofthese components.

The stator 14, a shell-like structure, encloses the rotor and comprisesa core 17 further comprising a plurality of thin, high-permeabilitycircumferential slotted laminations 17A placed in a side-by-sideorientation and insulated from each other to reduce eddy current losses.Stator coils are wound within the inwardly directed slots. The ACelectricity induced in the stator windings by action of the rotor'srotating magnetic field flows to terminals 19 mounted on the generatorframe for connection to an external electrical load. Three-phasealternating current is produced by a generator comprising threeindependent stator windings spaced at 120° around the stator shell.Single-phase alternating current is supplied by a stator having a singlestator winding.

The rotor 12 and the stator 14 are enclosed within a frame 20. Eachrotor end comprises a bearing journal (not shown) for cooperating withbearings 30 attached to the frame 20.

A generator cooling system removes heat produced by current flow throughthe generator conductors, including the direct current flow through therotor windings 13 and the alternating current induced in the statorcoils. Additional heat sources include mechanical losses, such aswindage caused by the spinning rotor, and friction at the bearings 30.The rotor 12 carries a blower 32 for forcing cooling fluid through thegenerator elements. Coolers 36 receive and cool the cooling fluid torelease the heat absorbed from the generator components. The coolingfluid is then recirculated back through the generator components.

To increase generator output and efficiency and reduce generator sizeand weight, conventional copper rotor windings are replaced bysuperconducting windings (filaments) that exhibit effectively noresistance to current flow when maintained below the material's criticaltemperature (T_(c)). Superconductivity is a phenomenon observed inseveral metals and ceramic materials when the material is cooled totemperatures ranging from near absolute zero (0° K. or −273° C.) to aliquid nitrogen temperature of about 77° K. or −196° C. The criticaltemperature for aluminum is about 1.19° K. and for YBa₂Cu₃O₇(yttrium-barium-copper-oxide) is about 90° K.Yttrium-barium-copper-oxide (one example of a high temperaturesuperconducting (HTS) material) is commonly used for the rotor windingsof a superconducting generator.

Since the superconducting materials exhibit substantially no electricalresistance when maintained at or below their critical temperature, thesematerials can carry a substantial electric current for a long durationwith insignificant energy losses. To maintain the superconductingconductors at or below their critical temperature, coolant flow pathscarrying coolant supplied from a cryogenic cooler are disposed adjacentor proximate the windings. Typical coolants comprise liquid helium,liquid nitrogen and liquid neon.

Disadvantageously, the HTS rotor windings are sensitive to mechanicalbending and tensile stresses that can cause premature degradation andwinding failure (e.g., an open circuit). For example, bends formed inthe HTS rotor windings to circumscribe the cylindrical rotor core inducewinding stresses. Normal rotor torque, transient fault condition torquesand over-speed forces induce additional stress forces in the rotorwindings. These over-speed and fault conditions substantially increasethe centrifugal force loads on the rotor coil windings beyond the loadsexperienced during normal operating conditions.

The co-pending commonly-owned application entitled Superconducting CoilSupport Structures (Attorney docket number 2006P13505US) describes andclaims HTS winding support structures that support the windings againstthese loads. This application is incorporated by reference herein. Thesupport structures also limit heat transfer from the “warm” (i.e.,approximately room temperature) rotor core to the “cold” (i.e.,cryogenically cooled) HTS windings. In addition to conductive thermalpaths in the support elements, it is desired to maintain the HTS rotorwindings in a vacuum condition to limit radiative heat transfer from therotor core to the superconducting windings.

AC electricity available at the stator terminals is supplied to anelectrical power grid through a transmission and distribution system.Grid fault currents, e.g. caused by a lightning-induced current spike onthe grid, are coupled to the stator through the intervening transmissionand distribution lines. The grid fault currents generate a stator faultcurrent and an attendant strong transient magnetic flux that ismagnetically coupled to the HTS rotor windings. This flux can generate asignificant torque on the rotor core and the HTS winding, potentiallydamaging the HTS winding and its support structures. The transientmagnetic fields can also be caused by system or internal short circuits,transmission switching operations, synchronizing operations, transientvoltages on the transmission system and loss of synchronism between thegenerator and the grid. In addition to the undesired mechanical forcesproduced by these transient torques, any magnetic field coupled into therotor windings causes undesired alternating current (AC) losses in theHTS conductors.

Although rotor winding support structures can be designed to allow theHTS conductors to withstand the additional torque introduced by thesetransient magnetic fields, such support structures increase the supportmass and may introduce additional undesired thermal paths between thewarm rotor core and the cold HTS windings.

Typically however, the rotor windings are shielded to prevent transientmagnetic fields from reaching the rotor HTS windings. An electromagneticshield, comprising copper or aluminum for example, encloses the HTSrotor windings to prevent magnetic flux from coupling to the rotor,thereby avoiding the consequent torques induced on the HTS windings. Theshield is also referred to as a non-magnetic shield since it isconstructed from non-magnetic material.

For relatively small electric generators the shield comprises a thintubular or cylindrical structure surrounding the rotor core and the HTSwindings and attached to the rotor core end faces. However, it is asubstantial challenge to manufacture, assemble and balance a large andcontinuous cylindrical shield structure with the precision andtolerances required for a large electrical generator. According to oneembodiment, a tubular shield having a relatively thin wall surface issupported by the rotor shaft with a tight clearance between the rotorand the shield. Gravity loading deforms the thin tube into an ellipticalshape and interface contact is made at the top and bottom surfaces ofthe rotor shaft. Further, the considerable rotor weight tends to causerotor sag. These effects lead to fretting damage due to relative motion(albeit a small displacement) at the interface of the rotor core and thenon-magnetic shield. Alternatively, the tube shield has relatively thickwall surface with a larger gap between the shield and the rotor. Littleor no fretting damage occurs in this configuration, but the shield mustbe sufficiently thick to support its own weight.

The rotor core and the surrounding non-magnetic shield independentlyvibrate at a different resonant frequency with a different vibrationpattern. These effects create additional dynamic loads on the rotor coreand the HTS windings. The cumulative effect of the interface contactforces and the vibration forces create extremely high stresses on therotating non-magnetic shield.

If the rotor core and the HTS rotor windings are enclosed in a vacuumvessel (comprising stainless steel for example) additional designdifficulties arise. If the shield and the vacuum vessel are bothcylindrical with the vacuum vessel nested within the shield they arepreferably joined to maintain the vacuum condition. Joining thedissimilar metals of the vacuum vessel and the shield is problematic.Further, the disadvantages associated with the large generator shielddiscussed above are exacerbated by the addition of the vacuum vessel.

It is known by those skilled in the art that the rotor must be balancedto minimize undesired rotor torques. During the balancing processbalancing weights are added to the rotor body at various locations alongits axial length to balance the rotor at its operating speed. Effectivebalancing requires access to the entire rotor body surface to permitplacement of the balancing weights as desired to effect a balancedcondition. A shield that covers the entire rotor requires performing thebalancing operation prior to placement of the shield over the rotor. Butsuch a process increases production cycle time and process costs. Also,this pre-shield installation balancing operation is conducted with therotor at ambient temperature, but the rotor operates at cryogenictemperatures. Undoubtedly, the lower temperature affects the rotor'sbalance. Thus it is preferable to balance the rotor under cryogenicoperating conditions with access to the entire rotor surface to placebalance weights as required.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the invention comprises a rotor for an electricgenerator. The rotor comprises a rotor core defining a first and asecond axially extending flat surface region, each of the first and thesecond flat surface regions bounded by first and second opposing edges;a superconducting winding circumscribing the rotor core, axial segmentsof the superconducting winding disposed within the first and the secondflat surface regions; a plurality of first and a plurality of secondcore extension elements mating with the respective first edge of thefirst and the second flat surface regions; a first arcuate shieldextending axially along the rotor core enclosing the axial segmentwithin the first flat surface region, the first shield having a firstaxial edge mating with a first edge of the plurality of first coreextension elements and a second axial edge mating with the second edgeof the first flat surface region and a second arcuate shield extendingaxially along the rotor core enclosing the axial segment within thesecond flat surface region, the second shield having a first axial edgemating with a first edge of the plurality of second extension elementsand a second axial edge mating with the second edge of the second flatsurface region.

In another embodiment the invention comprises a rotor shield for a rotorcore of an electric generator comprising a superconducting winding. Therotor shield comprises an arcuate sheet of electrically conductivenon-magnetic material for shielding the superconducting winding and eachone of a first and a second axial edge of the arcuate sheet having afirst dovetail feature for mating with a second dovetail feature of arespective first and a second core element.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will be apparent fromthe following more particular description of the invention, asillustrated in the accompanying drawings in which like referencecharacters refer to the same parts throughout the different figures. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

FIG. 1 is a cross-sectional view of a prior art electric generator;

FIG. 2 is an illustration of a rotor for use in a superconductingdynamoelectric machine according to the teachings of the presentinvention;

FIGS. 3, 4, 5A, 5B and 6 are perspective views of a rotor for use with ashield of the present invention.

FIGS. 7 and 8 are views of a non-magnetic shield constructed accordingto the teachings of the present invention.

FIGS. 9 and 10 are perspective views of a rotor including thenon-magnetic shield of the present invention.

FIG. 11 is an illustration of another embodiment of a non-magneticshield of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing in detail the particular non-magnetic rotor windingshield in accordance with the present invention, it should be observedthat the present invention resides primarily in a novel and non-obviouscombination of hardware elements and method steps. Accordingly, theseelements and steps have been represented by conventional elements andsteps in the drawings, showing only those specific details that arepertinent to the present invention so as not to obscure the disclosurewith details that will be readily apparent to those skilled in the arthaving the benefit of the description herein.

The following embodiments are not intended to define limits as to thestructures or methods of the invention, but only to provide exemplaryconstructions. The embodiments are permissive rather than mandatory andillustrative rather than exhaustive.

A rotor shield of the present invention overcomes the variouslimitations described above. The shield of the present inventioneffectively prevents stator-originating transient time-varying magneticfields from impinging rotor HTS windings, but allows access tosubstantially all of the rotor body for the placement of balancingweights. Also, the shield and the rotor body cooperate to transfertransient loads (generated during fault conditions) and steady stateloads imposed on the shield to the rotor body.

It is known that the shield provides a shielding function only formagnetic fields that vary in time at the surface of the rotor. Since theshield rotates with the rotor flux (which is generated by a DC current)there is no time-varying rotor flux component and thus the shield doesnot impede the main rotor flux. The shape of the rotor pole tends toreduce the magnetomotive force (MMF) drop of the rotor pole to a lowlevel and thereby ensures that the rotor leakage flux remains at arelatively low level.

However, the shield shields the HTS windings from stator magnetic fluxcomponents that are time-varying from the perspective of the rotor. Asis known, a time-varying field generates a time-varying voltage on thesurface of the conductive shield responsive to the change of magneticflux with time. A current flows within the shield responsive to thisvoltage and generates a time-varying magnetic field that counters theexternal time-varying magnetic field. Thus the time-varying field isprevented from reaching the rotor core and the HTS windings.

FIG. 2 illustrates a superconducting rotor 50 defining a longitudinalaxis 52 and comprising a generally cylindrically-shaped core 54 andcoaxially aligned rotor end segments 55 and 57 each attached to an endsurface of the core 54. A material of the core 54 exhibits a highmagnetic permeability, e.g. a ferromagnetic material such as iron, forincreasing the magnetic flux generated by the rotor windings.

The superconducting rotor 50 further comprises a generallylongitudinally-extending, racetrack-shaped superconducting (HTS) coil orwinding 60 comprising axial segments 60A connected by radial segments60B, the latter extending through openings 55A and 57A defined betweenend surfaces of the core 54 and the respective end segments 55 and 57.Non-magnetic shields 70A and 70B of the present invention are eachsupported by the rotor core 54 and enclose the superconducting coilsegments 60A.

The end segment 57 further comprises a cryogenic transfer coupling 68that supplies cooling fluid (cryogenic fluid) from a cryogenic cooler(not shown) to closed coolant flow paths or channels in thesuperconducting coil 60 to maintain the superconducting coil 60 at orbelow its critical temperature. From the channels, the coolant returnsto the transfer coupling 68 then to the cooler for lowering the coolanttemperature. The coolant is then circulated back to the coolant flowpaths.

The rotor 50 for use with the magnetic shield of the present inventionis illustrated in greater detail in FIG. 3, absent the rotor endsegments 55 and 57. The rotor core 54 comprises oppositely-disposedaxially-extending flat surface regions 404. The flat surfaces balancethe stiffness of the rotor to avoid excessive dynamic forces.

FIG. 4 illustrates the rotor core 54 and the superconducting windingsegment 60A supported by the aforementioned HTS winding supportstructures attached to the flat surface regions 404. As illustrated inFIG. 5A, a plurality of blocks 412 (also referred to as core extensionsand comprising a ferromagnetic material such as steel) are disposed in aside-by-side configuration axially along one exposed edge of each flatsurface region 404, with a spacer 413 intermediate two adjacent blocks.Typically, the blocks 412 are installed after the superconductingwinding 60 is attached to the core 54. In one embodiment the blockscomprise a dovetail surface 412A that mates with a correspondingdovetail groove in the rotor core 54. See FIG. 5B.

As can be seen, the blocks 412 partially close the circumferential coregap formed by the flat surface regions 404. The blocks 412 arefunctional elements of the core 50 (i.e., a material of the blocks 412comprises a ferromagnetic material) and thus are formed from a core-likematerial. The blocks 412 also support the magnetic shield of the presentinvention as described further below. The blocks 412 can be installedbeginning from either end of the core 50.

In lieu of individual blocks 412, the circumferential gap can be closedby a single elongated piece (formed from ferromagnetic material)extending a length of the rotor core 50.

FIG. 6 illustrates the partially assembled rotor core 54, including thesuperconducting winding segment 60A, the blocks 412 and the spacers 413,with end segments 55 and 57 affixed thereto according to knowntechniques.

FIGS. 7 (a perspective view) and FIG. 8 (an end view) illustrate oneembodiment of a non-magnetic shield assembly 424 constructed accordingto the teachings of the present invention, comprising an arcuate shield426 preferably constructed of aluminum (or another non-magneticmaterial). A plurality of adjacent sliding shoes 428 mate with theshield 426 at a dovetail interface along a shield edge surface 430. Aplurality of sliding shoes 432 similarly mate with an opposing edgesurface 434 of the arcuate shield 426. Each of the sliding shoes 428 and432 is attached to the shield 426 by a plurality of fasteners, such asbolts 435 as indicated in FIG. 8. In one embodiment, adjacent slidingshoes 428 and adjacent sliding shoes 432 are spaced apart to avoidfretting damage to the shoes or a spacer member is insertedtherebetween.

A dovetail surface 432A of the shoe 432 is received within a matingdovetail groove 438 in the rotor core 54. See FIGS. 8 and 9. A dovetailsurface 428A of the oppositely disposed sliding shoe 428 is similarlyattached (using a dovetail mating technique) to an exposed surface ofeach of the magnetic steel blocks 412.

To install the non-magnetic shield 426, the sliding shoes 428 and 432are affixed to the shield 426. The surfaces 432A and 428A are alignedwith respective mating grooves in the rotor core 54 and the magneticsteel blocks 412. The non-magnetic shield assembly 424 is then slidaxially along the rotor core 54 to cover and enclose the superconductingwinding portion 60A.

Similarly, a second non-magnetic shield is affixed to the rotor core 54to close the oppositely disposed flat surface region 404 and thesuperconducting winding portion 60A (see FIG. 2) affixed thereto.

As illustrated in FIG. 10, an end cap 440 is attached to the rotor core54 to close open ends formed when the non-magnetic shield assembly 424is in place on the rotor core. Another end cap is similarly situated atthe other end of the non-magnetic shield assembly 424. As can also beseen in FIG. 10, the cooperating the non-magnetic shield assembly 424and the end caps 440 completely enclose the super conducting windingportion 60A.

FIGS. 9 and 10 further illustrate bolts 450 for attaching the endsegment 57 to the core 54.

Use of the non-magnetic shield assembly 424 in lieu of a shield thatcompletely surrounds the rotor as known in the prior art, substantiallyreduces dynamic loads on the rotor core 54 and on the assembly 424during both steady state and transient load conditions, while shieldingthe HIS winding 60 from transient magnetic fields.

In another embodiment illustrated in FIG. 11, a magnetic shield 460comprises a plurality of side-by-side curved elements or bands 462extending axially along the rotor core 54. The elements 462 may bespaced apart, but electrical conductivity must be maintained between theelements 462.

While the present invention has been described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes may be made and equivalent elements may besubstituted for the elements thereof without departing from the scope ofthe invention. The scope of the, present invention further includes anycombination of elements from the various embodiments set forth herein.In addition, modifications may be made to adapt a particular situationto the teachings of the present invention without departing from itsessential scope. Therefore, it is intended that the invention not belimited to the particular embodiments disclosed, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1.-18. (canceled)
 19. A rotor shield for a rotor core of an electric generator comprising a superconducting winding, the rotor shield comprising: an arcuate sheet of electrically conductive non-magnetic material for shielding the superconducting winding; and each one of a first and a second axial edge of the arcuate sheet having a first dovetail feature for mating with a second dovetail feature of a respective first and a second core element.
 20. The rotor shield of claim 19 wherein the sheet comprises a plurality of arcuate bands arranged in a side-by-side orientation.
 21. The rotor of claim 19 wherein the first and the second core elements each comprises a first and a second axial edge bounding a flat surface region of the rotor core.
 22. The rotor core of claim 19 further comprising a first shoe intermediate the first axial edge of the arcuate sheet and the first core element, the first shoe affixed to the first axial edge and slidably engaged to the first core element, and the rotor core further comprising a second shoe element intermediate the second axial edge of the arcuate sheet and the second core element, the second shoe affixed to the second axial edge and slidably engaged to the second core element. 